1. Field of the Invention
This invention generally relates to a fluid compressor and, more particularly, to a helical blade type compressor for compressing a fluid such as a refrigerant gas in a refrigeration cycle, for example.
2. Description of the Related Art
A helical blade type compressor is one of closed compressors. A compressor of this type utilizes the principle of a fluid supply screw pump disclosed in U.S. Pat. No. 2,401,189. Such a compressor is disclosed in U.S. Pat. No. 4,871,304 assigned to the present assignee.
FIG. 10 shows a main part of a conventional helical blade type compressor as disclosed in the above application. This compressor is mainly constituted by a cylinder 1, a piston 2 which rotates relative to the cylinder 1 (eccentric rotational motion), and a blade 4 inserted in a groove 3 which is helically formed in the outer surface of the piston 2, as shown in FIG. 11. As the piston 2 rotates relative to the cylinder 1, the blade 4 slides in the helical groove 3. At this time, part of the blade 4 moves in and out of the groove 3 in a direction perpendicular to the axis the cylinder 1, and hence each top portion of the blade 4 is sequentially brought into contact with the inner wall of the cylinder 1. Both the ends of the cylinder 1 and the piston 2 are rotatably supported by bearings 5 and 6. Suction and discharge ports 7 and 8 are respectively formed in the bearings 5 and 6. The pitch of the helical groove 3 is gradually decreased from the suction port 7 to the discharge port 8.
An operation of the compressor will be described below. When relative rotational motion of the cylinder 1 and the piston 2 is started, a fluid to be compressed is drawn into a space (to be referred to as an operation chamber) between the cylinder 1 and the piston 2 through the suction port 7. Since the operation chamber is partitioned by the blade 4, the fluid is compressed. More specifically, as the operation chamber moves to the discharge port 8 side upon rotational motion of the piston 2 relative to the cylinder 1, since the pitch of the helical groove 3 is gradually decreased, the volume of the operation chamber is gradually decreased. Therefore, the fluid drawn in the operation chamber is gradually compressed and is finally discharged from the discharge portion 8.
In a compressor having such an arrangement, however, a thrust F due to a differential pressure always acts in a direction from the discharge port 8 side to the suction port 7 side. This thrust F causes a loss (to be referred to as a thrust loss) of the compressor.
In addition, since the pitch of the helical groove 3 formed in the outer surface of the piston 2 is gradually changed, large deformation/distortion of the blade 4 inserted in the groove 3 is generated. The deformation/distortion increases a slide loss caused when the blade 4 moves in and out of the groove 3, and makes the blade 4 to be susceptible to damage, resulting in a decrease in reliability.
As described above, in the conventional helical blade type compressor, a thrust loss due to the difference in pressure between the suction port side and the discharge port side and a slide loss due to deformation/distortion of the blade are increased, and the reliability is decreased.
Such problems lead to a decrease in efficiency as a compressor and in reliability. These problems adversely affect the compressor, especially, in terms of efficiency. For example, in a compact compressor receiving a low input of about 100 W, the above-described thrust corresponds to about 30 kg, and a thrust loss consumes 10 W of the input.